1. Field of the Invention
This invention relates to method and apparatus useful in connection with removing asbestos insulation from a hot surface, such as that of the wall of a large industrial boiler.
2. Commercial Prior Art
As stated, the invention has been developed in connection with asbestos removal from boilers. For purposes of illustration, it will be described in that connection. However, it is to be understood that use of the invention can be extended to other insulated hot surfaces, such as plant ducts and piping. There is therefore no intention to restrict the scope of the invention to the field of boilers.
The boilers, in connection with which the invention has been developed, are large industrial units. Typically, the boiler might have a base that measures 30'.times.30' and a height of 60'. It is insulated with a layer of asbestos which is contained by an outer skin of sheet metal cladding. The temperature at the outer surface of the boiler steel wall can vary between about 275.degree. F. and 700.degree. F. The temperature of the cladding might be 130.degree. F. Absent the cladding and insulation, the boiler obviously constitutes a potent source of radiant heat.
Since the 1960's, there has been an increasing awareness that asbestos creates a serious hazard to a human, if ingested. So plant operators having asbestos-insulated units are gradually removing the asbestos and replacing it with other insulating material.
Unfortunately, asbestos is a very difficult material to work with. The fibers are minute --they are too fine to be seen by the human eye. And they are readily airborne and can float in the air for days on end.
Heretofore, the removal of the asbestos from large industrial boilers has involved shutting down the unit and allowing it to cool, before initiating the asbestos removal process. We choose to refer to this conventional process as a "cold" removal process.
When the boiler is shut down for this purpose, there is naturally a loss of production in the plant as a whole. By way of example, in one forest products plant at which the present applicant carried out a cold removal, the lost production was valued at $4l,000/hour.
At this point, it is appropriate to describe the conventional cold removal process:
As a first step, hoarding, comprising a wood frame covered with polyethylene sheeting, is erected and sealed with tape around the boiler. The sheeting is secured to the wood frame with lath and staples. All openings in the hoarding are sealed with adhesive tape. In effect, an envelope incorporating a supporting floor is applied. However, there are apertures in the hoarding, which are controlled by flaps. When suction is applied to the work chamber formed by the hoarding, the flaps open to allow a limited amount of outside air to enter the chamber. However, if the negative pressure is lost, the flaps normally close. Suitable sealed doors are also provided for entry into the work chamber. The envelope so formed is adapted to prevent the escape of air out of the chamber. This is essential as the plant workers outside the hoarding are not protected with protective breathing apparatus; PA0 With the hoarding in place, a negative air machine assembly is provided, having one or more air outlets that are mounted in the hoarding wall. The negative air machine assembly functions to draw outside air into the chamber and to suction contaminated air from the chamber. This withdrawn contaminated air is filtered in three sequential stages in the assembly, to trap the fibers and discharge cleaned air. The negative air machine assembly is normally operated to maintain a negative air pressure of 0.05 to 0.08 inches of water in the chamber, relative to atmospheric pressure. The system is further typically operated to change the air in the chamber 4-5 times per hour; PA0 With this system in place and operating, the workers, properly clothed and equipped with breathing apparatus, enter the chamber and remove the cladding. They then scrape off the insulation while simultaneously soaking it with a stream of water. The sodden clumps of asbestos are immediately bagged as they are removed. A steel brush is then used to polish the steel surface while a vacuum hose is held close to the brush to catch loosened fibers. As a final step, the cold steel surface of the cleaned wall is washed down with water using a high pressure sprayer and then sprayed with a liquid sealant that solidifies over time to form a solid coating that fixes remaining fibers of asbestos still clinging to the steel. PA0 It is significant to note that wetting down the asbestos with water is an important part of the process, to keep fibers from becoming airborne. PA0 (2) It would not be possible to soak the asbestos with water, as it would turn into steam. Steam damages the filters used in the breathing apparatus and in the negative air machines. The removal of the asbestos would therefore have to be carried out on a dry basis. It then follows that the incidence of floating asbestos fibers would be accordingly much greater. PA0 (1) providing a relatively small, non-combustible, air-cooled hoarding, sealed to the hot clad wall, forming a chamber in which a human can safely and comfortably work in the course of removing asbestos; and PA0 (2) providing such a chamber and removing the asbestos in accordance with a specified procedure. PA0 (a) The hoarding is relatively small in comparison to the hoarding typically used with a cold removal. For example, in the case of a 30'.times.30'.times.60' boiler the cold removal hoarding would commonly encapsulate the entire unit. The chamber of such a large hoarding might take up 18,000 cu. ft. In the present hot removal system, the hoarding typically might form a rectangular chamber having a volume of 1600 cu. ft. (typically 8' high, 8' wide and 10' long). Stated otherwise, the hot removal hoarding only covers a minor portion or patch of the hot surface being worked on; PA0 (b) The hoarding is formed with a non-combustible innermost portion. Preferably, the noncombustible hoarding portion is formed of sheet metal and extends out about 3 feet from the cladding; PA0 (c) A fan assembly is provided, preferably having a vertically extending array of air supply inlets mounted in the hoarding wall at one end of the chamber. The fan assembly is adapted to inject cool air (preferably having a temperature below 50.degree. F.) into the chamber in sufficient amount to ensure that the wet bulb globe temperature ("WBGT") in the chamber is maintained at less than about 80.degree. F. PA0 (d) A suction assembly is provided at the other end of the chamber for withdrawing, cleaning and discharging air from the chamber while maintaining a negative air pressure therein, preferably in the order of 0.05 to 0.08 inches of water. Preferably, a battery of negative air machines is provided having a vertically extending array of suction outlets mounted in the hoarding wall at the other end of the chamber. The suction assembly is preferably operated to change the chamber air about 95 to 360 times per hour, as compared to the cold removal rate of 4 to 5 times per hour; PA0 (e) Each of the centers of the fan assembly inlets and suction assembly outlets are preferably positioned to ensure that the centre of the air flow is about 1 to 3 feet out from the cladding. The air supply inlets are directed to send a stream of cool air laterally across the face of the wall. Stated otherwise, a stream of cool air, extending across substantially the full vertical extent of the chamber, moves parallel to the boiler wall and the center of the stream is spaced out from the wall. PA0 It has been found that if the air stream is centered closer than about 1 foot from the cladding, excessive heat is picked up by the stream and the temperature within the chamber rises to unacceptable levels. If the stream is centered more than about 3 feet from the cladding, the workers, who stand about 2 feet from the cladding, are not sufficiently cooled by the stream, which is undesirable. PA0 It has also been found that the stream needs to move parallel to the wall, in spaced arrangement therewith. If the stream is directed against the wall, air turbulence results and excessive heat is extracted from the wall; and PA0 (f) With (a) to (e) in place, the workers remove part or all of the enclosed cladding and begin to dry remove the asbestos, preferably in narrow vertical strips (typically 3 feet in width), which are only a minor portion of the enclosed patch. As each strip is removed, a temporary insulating member, which can be a batt of mineral fibre insulation or a curtain of ceramic fiber cloth, is placed over the bared hot metal. As a result, the area of heat-emitting exposed steel wall is kept relatively small. PA0 Only a small volume chamber is involved, so that it becomes relatively easy to supply the desired high rates of air flow; PA0 The hoarding has been modified to cope with the hot near-wall conditions and will not burn, melt or lose its seal; PA0 The fact that dry removal of the asbestos will greatly increase the incidence of airborne asbestos has been coped with by massively expanding the rate of air flow, thereby maintaining the fiber concentration below acceptable leve ls when supplied air respirators are used by the workers; PA0 The fact that heat is being supplied into the enclosure has been dealt with by supplying a moving wall of cool air and learning how the air stream should be directed. The moving wall of air keeps the temperature within the chamber at the desired level, immerses the workers, and ensures that dead air "hot spots"do not develop; and PA0 The area of exposed heat source has been minimized by using the narrow strip removal technique and temporarily insulating the increasing area of cleaned steel with the insulating members.
It has long been recognized in the industry that it would be desirable to remove the asbestos without shutting down and cooling the boiler. (Hereinafter such a process is referred to as a `hot removal process`.) Prior to the present invention, to the best of applicants'knowledge, no commercially applied hot removal process had been developed.
The objective of the work underlying the present invention was to successfully develop a feasible hot removal process.
In setting out to develop such a process, applicants were faced with the following problems: p0 (1) Upon removing a patch of cladding and underlying asbestos, an enormous source of heat is exposed to radiate heat into the work chamber, with any of the following possible results:
The plastic could melt, PA1 The seals could be lost, PA1 The hoarding frame could catch fire, or PA1 The workers could be driven out by the heat; and
With this background in mind, the present invention will now be described.
When the word "airtight" is used herein to describe the hoarding, it is used in the sense that air should not be able to escape out of the chamber formed by the hoarding.